Plant-associated bacteria of Syringa vulgaris L. in an urban environment

Purpose Beneficial bacteria contribute significantly to host-plant adaptive capacities. Lilacs are common shrubs of urban areas in a northern climate. Their buds are the primary material for the Syringa aseptic micropropagation using a Murashige and Skoog medium (MS). As opposed to considering bud-associated bacteria as just contaminants, the search for plant growth-promoting bacteria (PGPB) provides a potential tool for better lilac-microbial co-cultivation. Methods In total, six cultivars of Siringa vulgaris were studied for the bacterial diversity of their roots and buds. By scanning electron microscopy, the localization of microorganisms was studied. The composition of bacterial communities was analyzed by Illumina Miseq new generation sequencing (NGS). Culturable bacteria of rhizoplane and surface-sterilized buds were isolated and compared. Indole-3-acetic acid (IAA) production under different cultivation conditions was studied colorimetrically among bud-associated bacteria capable of growing in MS medium. Results Pseudomonadota and Bacillota phyla dominated among cultivable bacteria; Proteobacteria, Actinobacteria, Firmicutes, Chloroflexi, Acidobacteriota, Verrucomicrobiota, Bacteroidota, and Crenarchaeota were detected by NGS. The endophytic communities differed at the genus level with dominating non-pathogenic Curtobacterium and Pseudomonas. Attached bacteria on the non-sterilized and surface-sterilized buds were observed. Bud-associated Methyllobacterium and Paenibacillus showed tryptophan-dependent biosynthesis of auxins. It strongly depended on the medium, with the lower IAA production in MS. Conclusion Lilacs maintain a wide diversity of root-associated bacteria with a number of beneficial strains. The endophytic community is limited and varies among the cultivars. Bud-associated PGPB may survive during sterilization and reveal their growth-promoting activity by producing auxins, though optimization of plant-microbial interactions is needed.